1. Field of the Invention
The present invention relates generally to downhole oil well tools namely run on a pipe string, impact or jarring type downhole oil well tools, and more particularly, to a fluid operated jarring tool for use in well bores that jars upwardly and downwardly and wherein the tool has a bit or working end that rotates when the bit is not subject to weight of the pipe string in order to prevent imprinting on the drilling surface.
2. General Background
In downhole well operation, there is a need for jarring or impact devices. For example, in workover operations using a pipe string such as coil tubing or snubbing equipment, it is necessary to provide downward jarring impact at the bottom of the string to enable the string to pass obstructions or otherwise enter the well. During fishing operations or other operations, such as paraffin scraping, it is sometimes necessary to apply upward jarring or impact forces at the bottom of the string if the fishing tool or the like becomes stuck.
In prior U.S. Pat. No. 3,946,819, naming the applicant herein as patentee, there is disclosed a fluid operated well tool adapted to deliver downward jarring forces when the tool encounters obstructions. The tool of my prior U.S. Pat. No. 3,946,819, generally includes a housing with a tubular stem member telescopically received in the housing for relative reciprocal movement between a first terminal position and a second terminal position in response to fluid pressure in the housing. The lower portion of the housing is formed to define a downwardly facing hammer and the stem member includes an upwardly facing anvil which is positioned to be struck by the hammer. The tool includes a valve assembly that is responsive to predetermined movement of the stem member toward the second terminal position to relieve fluid pressure and permit the stem member to return to the first terminal position. When the valve assembly relieves fluid pressure, the hammer moves into abrupt striking contact with the anvil. The tool of prior U.S. Pat. No. 3,946,819, is effective in providing downward repetitive blows. The tool of the '819 patent will not produce upwardly directed blows.
In prior U.S. Pat. No. 4,462,471, naming the applicant herein as patentee, there is provided a bidirectional fluid operated jarring apparatus that produces jarring forces in either the upward or downward direction. The jarring apparatus was used to provide upward or downward impact forces as desired downhole without removing the tool from the well bore for modification. The device provides downward jarring forces when the tool is in compression, as when pipe weight is being applied downwardly on the tool, and produces strong upward forces when is in tension, as when the tool is being pulled upwardly.
In U.S. Pat. No. 4,462,471, there is disclosed a jarring or drilling mechanism that may be adapted to provide upward and downward blows. The mechanism of the '471 patent includes a housing having opposed axially spaced apart hammer surfaces slidingly mounted within the housing between the anvil surfaces. A spring is provided for urging the hammer upwardly. When it is desired to use the mechanism of the '471 patent for jarring, a valve including a closure and a compression spring is dropped down the string to the mechanism.
In general, the mechanism of the '471 patent operates by fluid pressure acting on the valve and hammer to urge the valve and hammer axially downwardly until the downward movement of the valve is stopped, preferably by the full compression of the valve spring. When the downward movement of the valve stops, the seal between the valve and the hammer is broken and the valve moves axially upwarly.
The direction jarring of the mechanism of the '471 patent is determined by the relationship between the fluid pressure and the strength of the spring that urges the hammer upwardly. Normally, the mechanism is adapted for upward jarring. When the valve opens, the hammer moves upwardly to strike the downwardly facing anvil surface of the housing. The mechanism can be made to deliver a downward and upward blow by increasing the fluid pressure and decreasing the strength of the spring that urges the hammer upwardly. When the mechanism is so arranged, the downward momentum of the hammer is increased such that the hammer strikes the upwardly facing anvil of the housing prior to being urged upwardly to strike the downwardly facing anvil surface. The mechanism of the '471 patent can be adapted to produce only downward forces by either shortening the length of the valve spring or by lengthening the valve such that the valve recloses prior to the hammers reaching the downwardly facing anvil surface on the upstroke.
One of the problems with these prior art devices is the fact that during impact drilling, imprinting on the drilling surface can occur reducing or preventing penetration. The present invention rotates the working end, eg. a drill bit, during impact drilling. With the present invention, by rotating the bit when it is not subject to weight of the pipe string, very little energy is required. As compared to rotating the bit when it is weighted, this "unweighted" rotation slows bit wear. Thus, impact drilling can proceed with a constant movement or rotation of the bit to prevent imprinting on the drilling surface.